1. Field of the Invention
The present invention relates to a recording method and a reproducing method for an optical recording medium in which a nonvolatile data as well as a rewritable data can be recorded. Furthermore, the invention relates to an optical recording medium such as a magneto-optical recording medium and a phase-change type recording medium in which information recorded therein can be managed by using a nonvolatile data.
2. Description of Related Art
Various kinds of optical disks are now being used including a ROM optical disk such as a CD-ROM, a write-once optical disk in which an additional data is writable only once, and a rewritable magneto-optical disk. Such optical disks have been studied as core recording media in rapidly developing multi-media.
FIGS. 1 and 2 illustrate the principles of erasing and recording operations for a rewritable magneto-optical disk. As is shown in FIG. 1, a light beam irradiates a recording film 2 via an object lens 1, so as to heat the recording film 2 up to a temperature exceeding its Curie temperature. In cooling the recording film 2, the magnetization direction in the recording film 2 is aligned in accordance with an external magnetic field applied by a magnet 3, thereby erasing a recorded data. Furthermore, the direction of the external magnetic field applied by the magnet 3 is reversed as is shown in FIG. 2, and then a portion of the recording film 2 where the data is erased is selectively irradiated with a light beam via the object lens 1, thereby forming an area where the magnetization direction is reversed in accordance with a recorded data (i.e., forming a recording mark). Thus, a data can be recorded on the magneto-optical disk.
FIG. 3 is a diagram showing an exemplified configuration of an optical head used for such a magneto-optical disk. A magneto-optical disk 10 comprises a transparent substrate 11 and a recording film 2. A reference numeral 20 denotes a laser diode for emitting a light beam. On the light beam emitting side of the laser diode 20 are disposed, in this order, a collimate lens 21 for collimating a light beam emitted by the laser diode 20, a circular compensating prism 22 for making circular the section of the light beam from the collimated lens 21, a light beam splitter 23 for transmitting or reflecting the light beam, and an object lens 1 whose position is controlled by an actuator 24.
On the reflecting side of the light beam splitter 23 are disposed, in this order, a light beam splitter 25 for transmitting and reflecting an incident light beam, a half-wave plate 26 for rotating the deflection plane of an incident light beam, and a polarization light beam splitter 27 for separating an incident light beam into horizontal and vertical components. On the light beam exiting side of the polarization light beam splitter 27 are disposed photodetectors 28 and 29 for detecting the horizontal and vertical components of the light beam having passed through the polarization light beam splitter 27. The photodetectors 28 and 29 are connected to an amplifier 30 for amplifying a sum of detection signals of the photodetectors 28 and 29 and an amplifier 31 for amplifying a difference between the detection signals of the photodetectors 28 and 29. On the reflecting side of the light beam splitter 25 are disposed, in this order, a collective lens 32 for collecting an incident light beam, a cylindrical lens 33 and a quarter photodetector 34 for detecting the intensity of an incident light beam. The half-wave plate 26, the polarization light beam splitter 27 and the photodetectors 28 and 29 together form a magneto-optical signal detection system for detecting the change of the polarizing angle of an incident light beam caused by the magnetic Kerr effect due to a difference in the magnetization direction on the recording film 2. The collective lens 32, the cylindrical lens 33 and the quarter photodetector 34 together form a spot control signal detection system for focusing and tracking control of a light beam spot on the recording film 2.
A light beam emitted by the laser diode 20 is collimated by the collimate lens 21, and the collimated light beam is allowed to have a circular section by the circular compensating prism 22. Then, the resultant light beam is converged on the recording film 2 of the magneto-optical disk 10 via the object lens 1. A light beam reflected by the recording film 2 is reflected by the light beam splitter 23, and divided by the light beam splitter 25 into two directions respectively directed to the magneto-optical signal detection system and the spot control signal detection system.
In the magneto-optical signal detection system, the half-wave plate 26 makes the polarizing angle of the reflected light beam approximately 45.degree. against the polarization light beam splitter 27, so as to separate the light beam into the horizontal and vertical components by the polarization light beam splitter 27. The horizontal and vertical components are respectively converted into electric signals by the photodetectors 28 and 29. Based on a difference between the electric signals (obtained by the amplifier 31), a magneto-optical reproducing waveform which is reversed in accordance with the magnetization direction on the recording film 2 can be obtained. Furthermore, based on a sum of the electric signals (obtained by the amplifier 30), a signal corresponding to the change of the reflected light beam quantity is obtained, so as to reproduce a pre-format signal which is recorded on the magneto-optical disk 10 in the form of a pit. In the spot control signal detection system, based on a detection signal of the quarter photodetector 34, a focus error signal is obtained by using, for example, a known astigmatism method, and a tracking error signal is obtained by using, for example, a known push-pull method. Then, focusing and tracking of a spot is controlled based on these error signals.
Although a read-only data can be recorded in a pre-format portion of such a conventional magneto-optical disk, a write-once recording operation of a nonvolatile data cannot be conducted because data recorded in a general magneto-optical recording portion can be rewritten on principle.
Now, various known techniques for recording a nonvolatile data in a magneto-optical disk will be described. Japanese Patent Application Laid-Open No. 3-35446 (1991) discloses a magneto-optical disk comprising a recording layer used for a write-once recording operation as well as a recording layer used for a reversible recording operation. An irreversible write-once recording operation is conducted through irradiation with a light beam having a higher intensity than that used for a general recording operation, and the irreversibly recorded data is reproduced in accordance with the change of the reflected light beam quantity. Such a magneto-optical disk is required to have the additional recording layer for the write-once recording operation, and hence, a commercially available magneto-optical disk cannot be used for the write-once recording operation of a nonvolatile data.
Japanese Patent Application Laid-Open No. 59-71144 (1984) discloses a magneto-optical disk whose recording area is previously divided into a reversible recording area and an irreversible recording area (namely, the magneto-optical disk comprises a recording layer for the write-once recording operation similarly to the above-mentioned known technique), so as to selectively perform a rewritable recording operation and a nonvolatile recording operation. This magneto-optical disk has the same disadvantage as that of the aforementioned conventional technique, and additionally, the amount of nonvolatile data to be recorded is disadvantageously limited because the recording area is previously divided into the reversible and irreversible recording areas.
Japanese Patent Application Laid-Open Nos. 2-31356 (1990) and 56-119991 (1981) disclose a recording/reproducing apparatus which conducts an irreversible recording operation through crystallization of an amorphous magnetic film and a reproducing operation on the basis of the change of light quantity utilizing the change of a reflectance. When a data recorded in a user data area of a general magneto-optical disk is to be reproduced, an ID signal in a pro-format area is first read on the basis of the change of the light quantity, and the data in the user data area is subsequently read on the basis of the change of the polarizing angle. According to these conventional techniques, when a nonvolatile data is recorded in part of the user data area, a reproducing operation for an irreversibly recorded data in the user data area is different from that for a reversibly recorded data. Therefore, it is necessary to previously determine which part of the user data area is used as the irreversible recording area. As a result, the format of the disk becomes different from that of a general disk, and the reproducing operation of the recording/reproducing apparatus is to be partly changed. In addition, the irreversible recording area is restricted by the disk format, and hence, it is difficult for a user to change the size of the area.
Japanese Patent Application Laid-Open No. 5-290420 (1993) discloses a magneto-optical disk in which the direction of an axis of easy magnetization of the magneto-optical medium is changed from the vertical direction to the horizontal direction through irradiation with a light beam having a high power, so as to record a read-only signal utilizing the Kerr effect. The present inventors, however, have found through experiments that a reversed magnetization area is formed around a nonvolatile mark merely through the irradiation with a light beam having a high power, and that a reproducing signal cannot be obtained in the presence of such a reversed magnetization area.
The Applicant discloses, in Japanese Patent Application No. 6-223278 (1994), a method for forming a nonvolatile mark by irradiating a medium with a light beam having a higher power and/or rotating the medium at a smaller peripheral velocity than in the general recording operation. Also, it is found and described that a reversed magnetization area formed around the nonvolatile mark can be erased after the formation of the nonvolatile mark by a general erasing operation and that the nonvolatile mark can be reproduced by a general magneto-optical recording/reproducing method.
On the other hand, an optical recording medium has been widely acknowledged and spread as an external memory for a data processor such as a computer. This causes more and more risk of dishonest and illegal usage of a program and a data by copying them on another recording medium. A recording medium free from such illegal copy is described in Japanese Patent Application Laid-Open Nos. 60-175254 (1985) and 5-266576 (1993). Japanese Patent Application Laid-Open No. 60-175254 (1985) discloses a recording medium in which a defect as a label of a genuine product (hereinafter referred to as a medium identification code) is formed at a predetermined position. Only when the defect can be detected in a reproducing operation, a program recorded therein can be executed and a data recorded therein can be reproduced.
The recording medium disclosed in Japanese Patent Application Laid-Open No. 5-266576 (1993) includes a defect as the medium identification code formed in a predetermined sector, and the sector is recorded in an address table. When the sector including the defect is detected in a reproducing operation, the address table is referred to so as to determine whether the medium is a genuine product or a copy. In the case of illegal copy of a recording medium as described above, the copied program and data cannot be used. This prevents illegal copy of a recording medium, and a right to a program and a data therein can be protected.
However, in the aforementioned recording media, a defect on the recording media is detected as the medium identification code. Therefore, when a defect is accidentally formed on a recording medium, error detection can be caused. In such a case, a program cannot be executed and a data cannot be reproduced even though the recording medium is a genuine product. Furthermore, when the position of the defect is identified, a program and a data in a copied medium can be used by forming a defect at that position in the copied medium. In addition, when a defect is formed by providing a flaw on the surface of a medium, a recording film is exposed to air at the defect, so that the recording film can be oxidized and corroded. In such a case, recorded information can be eventually destroyed. Therefore, the reliability of the recording medium can be largely decreased.